This study is designed to test the hypothesis that the sensitivity of normal human tissues to radiotherapy is primarily due to the intrinsic radiosensitivity of cells, and that differences between individuals are basically genetic in origin and can be detected using tissue culture techniques. To test this, we propose to develop, from individual radiotherapy patients, cultures of lymphocytes, fibroblasts and keratinocytes and to assess each of them for chromosome sensitivity (breakage) and clonogenic survival following irradiation. The study will consist of three overlapping phases: 1. compare the sensitivity of three cell types derived from the same patient-- lymphocytes, fibroblasts, and keratinocytes; 2. determine the degree of variability in the sensitivity of lymphocytes, fibroblasts, and keratinocytes among different individuals; and 3. evaluate whether the in vitro radiosensitivity of one or more of these three cell types predicts the in vivo normal tissue response to radiotherapy. Patients to be included in this study will be radiotherapy patients who have experienced severe acute and/or late responses to radiotherapy, and patients with unknown normal tissue responses who will be sampled prior to treatment of defined head and neck cancer protocols with prospective monitoring of normal tissue reactions to be correlated with the in vitro radiosensitivity of their normal tissue cells.